Method and installation for making a workpiece comprising at least a tubular section

ABSTRACT

A method of forming a tubular workpiece which consists in pre-bending a metal plate along at least two longitudinal lines of bend ( 12–13 ) then in bending the plate along at least one of the lines of bend ( 12–13 ) using a bending member ( 103–104 ) that exerts an external clamping force (F 7 , F 8 ) along a part ( 23, 24 ) of the plate towards the at least one of the lines of bend ( 12, 13 ). Thereby preventing any sliding of part ( 23, 24 ) of the plate along the bending member ( 103–104 ) as the bending member bends the part. The bending member is articulated about a virtual geometrical axis (a, c) located inside a tubular section of the workpiece.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and to an installation for making aworkpiece comprising at least one tubular part obtained by bending ametal sheet. This method and this installation are particularly adaptedto the manufacture of a shadow mask support frame for a cathode-raydisplay tube.

2. Brief Description of the Related Art

Such a frame is used for example in the domain of the manufacture ofcolour televisions. The colour display cathode-ray tubes comprise ametal foil pierced with a plurality of holes or slots, called “shadowmask” and disposed between the electron gun and the display screen. Sucha shadow mask is supported by a frame, generally rectangular in shape,which holds it in position in the vicinity of the display screen and, ifnecessary, ensures that the mask is taut in order to limit thedeformations resulting from the local overheatings created by theelectron beams.

It is known from EP-A-0 809 272 to produce a shadow mask frame from twothin metal strips joined to each other and comprising ribs forrigidification. Such a frame is light and rigid, which allows it to beused for a taut shadow mask and for a crimped shadow mask. ApplicationFR-A-2 790 140 discloses a shadow mask frame comprising tubular partsformed by bending a metal sheet. The sheet may be bent by differentmethods, in particilar by hand, but a manual bending is not compatiblewith high-rate industrial production. Furthermore, the known methods ofbending with the aid of machines comprising an apron, sometimes called“support strap”, on which a metal sheet to be bent is held with the aidof a holding clamp, and an articulated bending flap, do not allowtubular parts to be easily manufactured.

SUMMARY OF THE INVENTION

It is an object of the present invention to propose a method of bendingwith the aid of a bending machine making it possible to manufactureworkpieces comprising at least one tubular part, particularly aworkpiece constituting one or more uprights of a frame supporting ashadow mask.

In this spirit, the invention relates to a method for making a workpiececomprising at least one tubular section obtained by bending a metalplate along at least one longitudinal edge, characterized in that itcomprises steps consisting in pre-bending the plate along at least twobending edges or lines of bending then in exerting, during bending, aclamping force along at least one part of the plate by a bending member.

With the invention, the metal plate is efficiently displaced by thebending member, without risk of sliding that may result in an imprecisebending along the edge or line of bending. Because of the pre-bending,the plate is maintained in position on an apron or supporting beam onwhich it rests, without risk of buckling during subsequent bending. Theforce of clamping or of holding of the plate with respect to the line ofbending makes it possible to hold the plate in position with respect toits environment without resorting to fixing clips which could be usedinside tubular parts only with difficulty, as they would hinder thebending operations.

According to advantageous but non-obligatory aspects of the invention,the method incorporates one or more of the following characteristics.

The pre-bending is effected by immobilizing the plate on an apron bymeans of a retractable holding clamp. In addition, a step of preparationof the pre-bending may be provided, by weakening the metal sheet alongthe lines of bending. Such weakening may be effected by marking orpunching through a part of the thickness of the metal sheet, or by anyother suitable method.

The plate is bent along different edges or lines of bending over thelength of the workpiece to be manufactured, with the result that tubularsections of different basic profiles are formed. This makes it possibleto produce workpieces of elaborate geometry, such as shadow mask supportframes.

The plate is bent about at least one virtual geometrical axis defined bythe cooperation of the bending member and of guiding means associatedtherewith.

The clamping force is essentially exerted on the outside of a tubularsection of the workpiece, in the absence of a tightening effort or forceexerted by a holding clamp or like equipment. This takes into accountthe fact that the use of a holding clamp is delicate with a tubularworkpiece, and even impossible if it has sections of different basicprofiles.

The clamping force is directed substantially in the direction of thebending edge or line of bending.

The clamping force is substantially perpendicular to the bending edge.

The clamping force stresses a part of the metal plate parallel to itselfand perpendicularly to the bending effort exerted on this part by thebending member.

The clamping force is adapted as a function of the position of thebending member. This aspect of the invention makes it possible to takeinto account the dimensional variations of the plate in the course ofthe bending operation, such variations resulting in particular from thelocalized stresses at the edge, or line of bending, and of its radius ofcurvature. It also makes it possible to avoid an interference betweenthe elements exerting a clamping force at two opposite edges, or linesof bending, of the plate.

The method further comprises a step consisting in causing a first partof the plate to overlap a second part thereof and in reducing theclamping force exerted on the second part before these parts are weldedtogether. This aspect of the invention ensures that the overlappingparts are in elastic contact during welding, which allows this weldingto be efficient and lasting.

The method also comprises a step consisting in bending the plate aboutat least one virtual axis defined as the geometrical axis of acylindrical cradle for guiding the bending member. The use of a virtualaxis makes it possible to move the bending member without resorting to ahinge capable of interfering with other parts of the installation anddedicated to a bending along another edge or line bending. Inparticular, it may be provided to bend the plate about different bendingaxes depending on the length of the workpiece, using bending membersguided by cradles of different geometrical axes corresponding to thesebending axes.

The method comprises a step consisting, after formation of a pluralityof tubular sections, in shaping them into a closed frame by bendingjoining sections, between two adjacent tubular sections, perpendicularlyto the principal directions of these tubular sections.

The method comprises a step consisting in applying, by suction ormagnetic attraction, a part of the sheet metal plate against a part ofthe bending member.

The invention also relates to an installation for carrying out themethod described hereinabove and, more specifically, to an installationcomprising at least one member for bending a metal plate along alongitudinal edge or bend line of a tubular section of the frame,characterized in that the bending member is equipped with means forexerting on a part of the plate a clamping force countering a sliding ofa part of the plate along the bending member, while the bending memberis articulated about a virtual geometrical axis located inside thecorresponding tubular section.

According to advantageous but non-obligatory aspects of the invention,the installation incorporates one or more of the followingcharacteristics.

A plurality of bending members are adapted to bend the plate alongdistinct lines or edges, with the result that tubular sections ofdifferent basic profiles are formed.

The clamping means are constituted by at least one heel forming a stopfor abutment of the plate. This heel may extend over substantially thewhole length of the line or edge. It is also possible to provide aplurality of heels distributed over the length of the bending line oredge and separated by spaces for receiving holding heels used forbending the plate along another line or edge.

The clamping means have a variable position with respect to the bendingline or edge. This may come from the fact that the installationcomprises means for adjusting or adapting the effort exerted by theclamping means on the plate.

The bending member comprises at least one surface for bearing againstthe plate and at least one circular-base cylindrical surface adapted tocooperate with a circular-base cylindrical surface of a cradle, thesecylindrical surfaces being centered on a virtual axis of bend of theplate. The use of such a cradle makes it possible to guide the bendingmember without resorting to a hinge that could interfere with adjacentdevices. In that case, the installation may be provided to comprise aplurality of bending members distributed over the length of the plateand cooperating with cradles of which the cylindrical surfaces haveparallel geometrical axes offset with respect to each other. This makesit possible to bend the plate along distinct longitudinal lines oredges, over its length, which enables tubular sections of differentgeometries to be shaped, corresponding for example to distinct sides ofa frame to be made. The bending member or members are advantageouslyequipped with an outer rib forming a cylindrical surface adapted tocooperate with the cylindrical surface of the corresponding cradle,these cylindrical surfaces being substantially of the same radii.According to an advantageous embodiment of the invention allowingtippings of great amplitude, the bending member or members are guidedwith respect to the cylindrical surface of the cradle by telescopicsegments which slide in one another. The surfaces of the ribs, thetelescopic segments and/or the surfaces of the cradles may be equippedwith sliding balls or rollers.

The installation comprises a tool for shaping the workpiece, the toolincluding elements for supporting tubular sections of the workpieceprovided with means for receiving and immobilizing these tubular parts,these elements being articulated with respect to one another. This toolallows the final shaping of the frame after the tubular parts have beenproduced. At least one of the articulated elements may be provided to beadjustable in length, which makes it possible to adapt the tool to themanufacture of frames of different dimensions. The articulated elementsmay also be provided to be adapted to form with one another a closedfigure of predetermined geometry, corresponding to the geometry of aframe formed by the manufactured workpiece. The closed nature of thefigure obtained makes it possible to obtain an increased geometricalprecision thanks to a universal positioning of the different elementsconstituting the tool. According to an advantageous form of embodimentof the invention, at least one of the articulated elements is equippedwith a member, of cross-section substantially in the form of a U andadapted to receive at least one shim for blocking a tubular section inplace in the member, means being provided to immobilize the shim on thismember.

At least one of the bending members is equipped with a blade formingshears for cutting out the plate during bending thereof.

Finally, the invention relates to the use of the method or of theinstallation mentioned hereinabove for manufacturing a shadow masksupport frame for a cathode-ray tube.

At least one of the bending members is equipped with a blade formingshears for cutting out the plate during bending thereof.

Finally, the invention relates to the use of the method or of theinstallation mentioned hereinabove for manufacturing a shadow masksupport frame for cathode-ray tube.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood and other advantagesthereof will appear more clearly in the light of the followingdescription of two forms of embodiment of an installation formanufacturing a shadow mask support frame and of its method ofimplementation, given solely by way of example and made with referenceto the accompanying drawings, in which:

FIG. 1 schematically shows a shadow mask frame on which is disposed apartially shown shadow mask;

FIG. 2 schematically shows a perspective view of a part of aninstallation for manufacturing the frame of FIG. 1, the frame not havingbeen shown;

FIG. 3 schematically shows a part of the frame of FIG. 1 when it is inplace in the installation of FIG. 2;

FIG. 4 is a section along line IV—IV in FIG. 2, the frame being shown inplace in the installation;

FIG. 5 is a section similar to FIG. 4 during a first step of the methodof the invention, prior to the configuration of FIG. 4;

FIG. 6 is a section similar to FIG. 4 during a subsequent step of themethod of the invention, later than the configuration of FIG. 4;

FIG. 7 is a section similar to FIG. 4 during a second subsequent step ofthe method of the invention

FIG. 8 is a section along line VIII—VIII in FIG. 2, the frame beingshown in place in the installation;

FIG. 9 is a section similar to FIG. 8 during a subsequent step of themethod of the invention;

FIG. 10 is a section similar to FIG. 8 during a second subsequent stepof the method of the invention;

FIG. 11 is a view in perspective of another part of the installation ofthe invention, a frame in the course of manufacture being shown abovethis part of the installation;

FIG. 12 is a view on a larger scale of detail XII in FIG. 11, the framebeing in place on the tool;

FIG. 13 is a view similar to FIG. 12 during a subsequent step ofmanufacture of the frame;

FIG. 14 is a plan view of the part of the installation shown in FIGS. 11to 13, in the course of use;

FIG. 15 is a partial schematic representation in section of a part of aplate in the course of bending;

FIG. 16 schematically shows a perspective view of a part of theinstallation;

FIG. 17 is a schematic section of a part of an installation inaccordance with a second form of embodiment of the invention;

FIG. 18 is a view similar to FIG. 13 for the installation of FIG. 17,and

FIG. 19 is a section along line XIX—XIX in FIG. 18.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically shows a shadow mask 1 mounted on a shadow masksupporting frame 2. The frame is generally rectangular in shape andcomprises two lateral uprights 3, 3′ and two end uprights 4, 4′. Theshadow mask is constituted by a thin metal foil, for example ofiron-nickel alloy with a low coefficient of expansion, pierced withholes 5 and fixed by welding on upper edges 6, 6′ of the end uprights 4,4′. Different forms of shadow mask are possible as a function of thecathode-ray tube with which it is to be associated. In the exampleshown, the shadow mask is substantially planar, but it may equally wellbe curved into a portion of cylinder. The mask I may in addition betaut, i.e. subjected to a tension parallel to the lateral uprights 3, 3′of the frame 2.

The uprights 3, 3′, 4 and 4′ are formed by bending a sheet metal so aseach to constitute a tubular element. These four uprights may beobtained by bending a single metal plate. It is also possible to provideusing one bent plate for each upright or one plate for two contiguousuprights.

In the example illustrated in the Figures, a single metal plate 10 isused for making the four tubular uprights 3, 3′, 4, 4′. This plate ispartially visible in FIG. 3 after steps of cutting out assembly tongues11 and of pre-marking longitudinal edges 12 to 17.

At the level of the edges 12 to 14, the plate 10 forms the end upright4. At the level of edges 12 and 15 to 17, the plate 10 forms the lateralupright 3. In practice, the part of the plate 10 shown in FIG. 3corresponds to the corner 7 of the frame 2 at the level of which thejoin is made between the uprights 3 and 4.

The plate 10 is shaped in the configuration of FIG. 3 by operations ofcut out, stamping and bending using, in particular, punches intended tocreate in the plate 10 an edge 12 extending over substantially the wholeof its length while the edges 13 to 17 extend over only a part of theplate 10, the edges 13 and 15 being parallel to each other but offset,i.e. distant from the edge 12 by different distances d₁ and d₂.

As is more particularly visible in FIG. 5, the pre-bending of the plate10 is effected by immobilizing this plate on an apron or support strap101 thanks to a holding clamp 107 and by bending the plate 10 asrepresented by arrows F, which has the effect of creating the lines ofbend 12 and 13. Another holding clamp is used at the level of theupright 3, which makes it possible to continue the creation of the lineof bend 12 and to create the line of bend 15. This second holding clampis also configured in order to allow the creation of the lines of bend16 and 17.

These operations of pre-bending make it possible to attain theconfiguration of FIG. 4 in which the holding clamp 107 is retracted orwithdrawn from the interior space of the plate 10 as represented byarrow F′. It will be noted that the shape of the holding clamp 107 iscompatible with its withdrawal from the interior of the plate 10 in theconfiguration of FIG. 4.

Thanks to this operation of pre-bending, the plate 10 is immobilizedwith respect to the installation of the invention, as it is in abutmentalong the two bending edges 12 and 13 or 12 and 15 which aresubstantially parallel.

Pre-bending may be prepared by a marking of the lines of bend, forexample by a stamping through a part of the thickness of the plate 10,in particular of 25%.

In general, the pre-bending of the plate 10 may be effected on themachine shown in the Figures or on another machine, without departingfrom the scope of the present invention.

In the event of an upright being made whose section has more than threesides, such as upright 3, preliminary pre-bendings are necessary inorder to make the edges of the type of edges 16 and 17. Such apreliminary pre-bending is also used in the case of the upright 4 formaking the edge 14.

The installation 100 of the invention comprises the apron 101 on whichis placed a central part 21 of the plate 10. Studs, of which only one isvisible in the Figures with reference 102, extend from this plate 101and are intended to traverse openings 22 provided in the part 21, so asto position the plate 10 on the base plate 101.

From the part 21 there extend two flanges 23 and 24 of the plate 10intended to form the upright 4. From this same same central part 21there extend two other flanges 25 and 26 intended to form the upright 3.

The flanges 23 and 25 are connected to the part 21 along the edge 12.The flanges 24 and 26 are respectively connected to the part 21 alongthe edges 13 and 15. The flange 23 is connected to a border 27 by theline of bend or edge 14 while the flanges 25 and 26 are both dividedinto two panels 25 a, 25 b and 26 a, 26 b respectively, by the lines ofbend or edges 16 and 17.

The installation 100 comprises a bending jaw 103 intended to bend theflange 23 down in the direction of part 21 thanks to an effortrepresented by arrow F₁. The jaw 103 comprises plate 103 a whose length,parallel to a longitudinal axis X–X′ of the installation 100, is adaptedto the length of the flange 23. The face 103 a 1 of the plate 103 aintended to be in contact with the flange 23 is substantially planar.The plate 103 a is equipped, on its face 103 a 2 opposite the face 103 a1, with two ribs 103 b extending opposite the flange 23 in directionssubstantially perpendicular to axis X–X′. These ribs 103 b each form aconvex surface 103 c in the form of a portion of cylinder with circularbase centred on a virtual axis a. “Virtual” is understood to mean thefact that the axis a is not defined by a hinge, but as being thegeometric focus joining the centres of curvature of the surfaces 103 c.This axis is located inside the upright 4 and is parallel to axis X–X′.

The installation 100 also comprises a cradle 113 defining a cylindricalinner surface 113 c and with circular base against which the ribs 103 bare in sliding abutment by their surfaces 103 c. The axis of symmetry ofthe surface 113 a is also the axis a. The radii of curvature R₁₀₃ andR₁₁₃ of the surfaces 103 c and 113 c are equal. It is thus possible topivot the jaw 103 in the cradle 113 about the axis a, as represented byarrow F₂ in FIG. 4, by relative slide of the surfaces 103 c and 113 c.

A second jaw 104 comprises a plate 104 a intended to come into abutment,by a face 104 a 1, against the flange 24 and to exert thereon an effortF₃ making it possible to bend it down in the direction of the base plate101 and of the central part 21. The plate 104 a is provided, on its face104 a 2 opposite the face 104 a 1, with ribs 104 b of which an outer orconvex surface 104 c is cylindrical with circular base and adapted tocome into abutment against an inner or concave cylindrical surface withcircular base 114 c of a cradle 114. c denotes the common centralgeometrical axis of the surfaces 104 c, this axis being parallel to axisX–X′ and located in the interior volume of the upright 4. 114 c and R₁₀₄and R₁₁₄ denote the respective radii of the surfaces 104 c and 114 c,these radii being of the same length. The jaw 104 may pivot about theaxis c being guided by the cradle 114, as represented by arrow F′₂.

Referring more particularly to FIGS. 4, 6 and 7, it will be understoodthat it is possible to shape the plate 10 in a tubular section withsubstantially triangular base thanks to efforts F₁ and F₃.

However, from the position of FIG. 4, if the efforts F₁ and F₃ areexerted while the flanges 23 and 24 rest respectively against the plates103 a and 104 a without any other effort, the plate 10 risks rising inthe direction of arrow F₄ in FIG. 4, because the flanges 23 and 24 risksliding along the surfaces 103 a 1 and 104 a 1, as respectivelyrepresented by arrows F₅ and F₆. This might result in an imprecisebending at the level of the edges 12 and 13 of which the centre ofcurvature would in that case no longer merge with the geometrical axes aand c.

In order to avoid relative movements between the flanges 23 and 24 andthe surfaces 103 a and 104 a, the jaws 103 and 104 are each equippedwith a heel 103 f, 104 f adapted to exert on the flanges 23 and 24 aneffort, respectively denoted F₇ and F₈, directed substantially towardsthe edges 12 and 13, i.e. countering a slide of the flanges 23 and 24 inthe direction of arrows F₅ and F₆. The efforts F₇ and F₈ thereforeconstitute efforts of clamping or of holding of the flanges 23 and 24with respect to the jaws 103 and 104. In particular, the flanges 23 and24 do not risk projecting radially outside the jaws 103 and 104.

In other words, the heels 103 f and 104 f form stops to the displacementof the flanges 23 and 24 in the direction of arrows F₅ and F₆.

It will be noted that the efforts F₇ and F₈ are exerted by the outsideof the upright 4. In this way, the plate 10 is maintained on the apron101 without using an added element, such as a holding clamp, this beingfavourable as a holding clamp would be delicate to place in position andto remove once the plate is bent. In addition, a holding clamp could notbe extended up to the level of the upright 3.

The invention therefore allows a bending of the plate 10 into a tubularstructure without introduction of a wedging device inside its differentsections.

The heel 103 f is in one piece with the plate 103 a, this heel beingintended to cover the border 27 of the flange 23. This heel 103 fextends over the whole length of the jaw 103 parallel to axis X–X′,which guarantees that the upper edge of the upright 4, on which the mask5 is intended to be stretched, is bereft of irregularities. To that end,the heel 103 presents a regular surface towards the border 27 as theheel 103 f serves for forming this border which must be as regular aspossible for the mask 1, which is stretched on the upper edge of theupright 4, not to present a wave or element in relief capable ofdisturbing the image generated in the cathode-ray tube.

The 104 f, which extends over the length of the jaw 104, is capable ofmoving perpendicularly to the plate 104 a and to the flange 24, asrepresented by arrow F₉ in FIG. 4.

Functioning is as follows:

When the plate 10 has been positioned on the apron 101 and pre-bentthanks to the holding clamp 107, the jaw 104 is displaced by apneumatic, electric or hydraulic jack, with the result that, thanks tothe effort F₃ exerted by the plate 104 a, it bends the plate 24 down inthe direction of the part 21. The heel 104 f exerts on the flange 24 aneffort F₈ directed towards the edge 12, which guarantees a correctpositioning of the flange 24 with respect to the jaw 104. The positionof FIG. 6 is then attained, where the angle α between the flange 24 andthe part 21 is less than 90°. From this position, the jaw 103 isactuated so that it bends the flange 23 down in the direction of thepart 21 by exerting the effort F₁, as shown in FIG. 7. The jaws 103 and104 are guided in their displacements F₂, and F′₂ thanks to thecooperation of the convex cylindrical surfaces 103 c, 104 c and concaveones 113 c, 114 c.

In order to avoid an interference between the heels 103 f and 104 f, theheel 104 f is moved away from the flange 24, as represented by arrow F′₉in FIG. 7. In effect, it is possible to eliminate the effort F₈ in theconfiguration of FIG. 6 as no displacement of the flange 24 is necessarybetween the configurations of FIGS. 6 and 7. Moreover, the fact that theangle α is less than 90° ensures an efficient positioning of the flange24, including in the event of elimination of the effort F₈.

In other words, in the configurations of FIGS. 6 and 7, the effort F₈ isnot indispensable as the plate 104 a efficiently holds the flange 24 inposition with respect to axis c.

According to a variant embodiment of the invention, it is possible toprovide for a plurality of heels 104 f to be distributed over the lengthof the plate 104 a, parallel to axes X–X′ and c.

Reference will now be made to FIG. 15 which is an enlargement of thedetail XV in FIG. 4. The plate 10 has a non-zero thickness e. Thepassage from the position shown in solid lines to the position shown indashed and dotted lines, which corresponds substantially to that of FIG.7, has the effect of reducing the height h of the flange 23 in theconfiguration in dashed and dotted lines with respect to the length l ofthis flange in the solid line configuration as the neutral axis f of theplate 10 forms an arc of circle centred on the axis a in theconfiguration shown in dashed and dotted lines. If it is considered thataxis a is located inside the zone intended to form the edge 12 with adistance d with respect to the neutral axis f and taking the example ofa plate curved from a planar configuration to arrive at a configurationbent at 90°, the difference between the height h and the length l is ½πd. In the present case, it is less insofar as the edge 12 is alreadypre-marked in the configuration of FIG. 4. However, it is not zero.

Taking into account this difference in values between the height h andthe length l, the value of the effort F₇ should be adapted so that itefficiently immobilizes the plate 10 in the zone of the edge 12 allalong the movement of rotation of the jaw 103 about axis a. To that end,the heel 103 f is provided to be elastically loaded in the direction ofaxis a or of the edge 12. In that case, the heel 103 f is not in onepiece with the plate 103 a. According to another approach, a cam systemmay be provided to allow the intensity of the effort F₇ to be varied asa function of the angular orientation of the jaw 103 about the axis a,in the course of the pivoting F₂.

According to another approach, it is possible to compensate thisdifference in value between the height h and the length l by offsettingthe pivot axis a in a direction perpendicular to the principal plane ofthe apron 101. In that case, the heel 103 f may be in one piece with theplate 103 a of the jaw 103.

Of course, the heel 104 f may also be provided with means for varyingthe intensity of the effort F₈ as a function of the position of jaw 104about axis c in the course of pivoting F′₂. In a variant, the positionof the axis c may also be provided to be variable in order to compensatethe variations in length and/or height of the flange 24.

Referring more particularly to FIGS. 8 to 10, it is noted that thelateral upright 3 is of quadrangular section. A jaw 105 is provided tocooperate with the flange 25 and comprises a base plate 105 a of whichone face 105 a 1 in contact with the flange 25 makes it possible toexert an effort F₁₁ to bend the flange 25 down in the direction ofelements 21 and 101. Like jaw 103, the jaw 105 is equipped with ribs 105b defining cylindrical convex surfaces 105 c centred on the axis a andmaking it possible to cooperate with a cradle 115 defining a concavesurface 115 c for slide of the surfaces 105 c, the surface 115 c being,like surfaces 105 c, of circular base and centred on axis a. The jaw 105is equipped with a plurality of heels 105 f distributed over its lengthparallel to axis X–X′, these heels being provided to come into contactwith the flange 25 at the level of edge 17, constituting a stop for apossible slide of the flange 25. The heels 105 f exert on the flange 25a clamping effort F₁₂ directed towards the edge 12 and parallel to panel25 a. The effort F₁₂ has the same function as the efforts F₇ and F₈evoked previously. It makes it possible to efficiently position theflange 25 with respect to the axis a all along the operation of bendingeffected thanks to the effort F₁₁.

In the same way, a jaw 106 is provided to cooperate with the flange 26and comprises a plate 106 a forming a face 106 a 1 for abutment againstthe flange 26 and equipped with ribs 106 b whose convex outer radialsurfaces 106 c cooperate with a concave surface 116 c of a cradle 116.The surfaces 106 c and 116 c are centred on a geometrical axis orvirtual axis b constituting the centre of the edge 15 and located, likeaxis a, in the interior volume of the upright 3.

The jaw 106 is equipped with heels 106 f making it possible to retainthe flange 26 in position with respect to the plate 106 a in the courseof bending of the edge 15. The plate 106 a is not planar but forms aconcave zone for receiving the panels 26 a and 26 b, the plate 106 aitself being formed by two panels oriented one with respect to the otherwith an angle β corresponding to the angle of orientation of the panels26 a and 26 b with respect to one another. The heels 106 f are disposedin order to come into abutment against the free edge 26 c of the panel26 b.

The heels 106 f are distributed over the length of the plate 106 a.

The heels 105 f on the one hand and 106 f on the other hand, arepositioned in quincunx with respect to each other, being offset alongthe axis X–X′, a heel 105 f being opposite a free gap 106 g definedbetween two heels 106 f, while a heel 106 f is disposed opposite a freegap 105 g defined between two heels 105 f.

The bending of the upright 3 may be explained with reference to FIGS. 8to 10. In the configuration of FIG. 8, an effort is exerted on the jaw105 by any appropriate means, with the result that it is displaced andexerts the bending effort F₁₁ of the flange 25 in the direction of thepart 21, the heels 105 f overlapping the zone of join 25 c, definedbetween the panels 25 a and 25 b and which includes the edge 17. Theeffort F₁₁ makes it possible to bend the flange 25 down as far as theposition of FIG. 9 where the panel 25 a is substantially perpendicularto the part 21, the panel 25 b being substantially parallel to thispart. In the course of the bending operation made between theconfigurations of FIGS. 8 and 9, the effort F₁₂ efficiently retains theflange 25 in position with respect to the jaw 105 and to the edge 12.

The jaw 106 is in that case activated, in order to pass from theposition of FIG. 9 to that of FIG. 10, an effort F₁₃ of bending of theflange 26 in the direction of the part 21 being exerted by the plate 106a while an effort F₁₄ is exerted by the heels 106 f on the flange 26parallel to this flange and substantially in the direction of the edge15.

As previously, the efforts F₁₂ and F₁₄ may be modulated as a function ofthe orientation of the jaws 105 and 106 about axes a and b. In avariant, the axes a and b may be offset with respect to theirrepresentation in FIGS. 8 to 10 in order to compensate the variations inlength of the flanges 25 and 26.

In addition, in the configuration of FIG. 10, the effort F₁₂ may bereleased or reduced by a movement of the heel 105 f in a sense of movingaway with respect to the edge 12, with the result that, due to theelasticity of the plate 10, the panel 25 b comes into firm elasticabutment against that face of the panel 26 b oriented towards the part21. It is in that case possible to spot weld the panels 25 b and 26 bthanks to a high energy beam directed between two adjacent heels 105 fand 106 f.

In order to attain the elastic abutment of the panels 25 d and 26 d, itis also possible to provide for the angle of bend between the panels 25a and 25 b to be slightly more open than that obtained in theconfiguration of FIG. 10, with the result that an elastic abutment isobtained in this configuration without it being necessary to release theeffort F₁₂.

As is more clearly visible in FIGS. 11 to 14, the frame 2 is conformed,after shaping of the different uprights 3, 3′, 4 and 4′, by bending.When the tubular uprights have been made, the plate 10 extendssubstantially in the direction X–X′ in four sections joined by zones ofjoin 8, 8′ and 8″. A tool 20 is provided for shaping the frame 2 andcomprises four flaps 201, 202, 203 and 204 intended each to receive oneof the uprights 3, 3′, 4 or 4′.

These flaps 201 to 204 are each formed by two massive pieces 201 a, 201b, 202 a, 202 b, 203 a, 203 b, 204 a, 204 b joined by a screw/nut system201 c, 202 c, 203 c or 204 c for adjustment of the flaps 201 to 204 inlength. Guide rods 201 c′, 202 c′, 203 c′ and 204 c′ are associated withthe systems 201 c to 204 c. The length of each flap 201 to 204 isadjusted, parallel to its largest dimension X₁ to X₄, to the length ofthe upright that it must receive. The flaps 201 to 204 are provided tobe adjustable in length in order to allow frames 2 of different sizes tobe manufactured.

The pieces 201 a and 201 b are respectively provided with angles 201 d,201 e for receiving the upright 4. Jaws 201 f, 201 g are provided toimmobilize the upright 4 against the angles 201 d and 201 e. In the sameway, the flaps 202, 203 and 204 are respectively equipped with angles202 d to 204 e and with jaws 202 f to 204 g. As is more particularlyvisible in FIGS. 11 and 12, the jaws are shaped as a function of thegeometry of the flanges 24 and 26.

The flaps 201 and 202 are articulated together about an axis Y₁essentially perpendicular to the principal direction X–X′ of theuprights 3, 4, 3′ and 4′. The axis Y₁ is also perpendicular to thedirection X₁ along which the flap 201 is adjustable in length. In thesame way, the flaps 202 and 203 are articulated together about an axisY₂ while the flaps 203 and 204 are articulated together about an axisY₃, these axes Y₂ and Y₃ being substantially perpendicular to axis X–X′and to the directions X₁, X₂, X₃ and X₄ for adjusting the differentflaps in length.

Three jacks 205, 206 and 207 respectively join the flaps 201 and 202,the flaps 202 and 203 and the flaps 203 and 204, with the result thatthey are adapted to exert thereon efforts or forces of closurerepresented by arrows F₁₆, F₁₇ and F₁₈ in FIG. 14. These efforts make itpossible to take the tool 200 into a substantially rectangularconfiguration in a plane perpendicular to axes Y₁ to Y₃, which makes itpossible to bend the frame 2 at zones 8, 8′ and 8″ and to form cornerssuch as corner 7, as represented in FIGS. 12 and 13. In effect, theeffort or force F₁₇ has the effect of bringing together the sides of theflaps 202 and 203 joined by the jack 206, which results in a bending ofthe plate 10 along a line 18 parallel to axis Y₂. The tongues 11arranged on the upright 4 are thus made to overlap the edge 3 a of theupright 3, these tongues and this edge in that case being able to bewelded by a high energy beam.

In the configuration of FIG. 14, the tool 200 forms a rectangle whoseflaps 201 to 204 constitute the four sides. This rectangle is closed atthe level of a zone of join between the flaps 201 and 204 where thereare provided locking means (not shown) associated with guiding means,likewise not shown. In this way, the rectangle shown in FIG. 14 isdefined precisely by the cooperation of the flaps 201 to 204, with theresult that the geometry of the frame 2 thus obtained is also definedprecisely.

According to an advantageous aspect of the invention shown solely inFIG. 16, the jaw 106 is equipped with a blade 106 h forming shears withthe plate 101 and making it possible to cut out, during the bending ofthe upright 3, the plate 10 in the zone of join of the uprights 3 and 4so as to create a notch 19 visible in FIG. 11 compatible with themovement of bend about axis Y₂. Due to the use of the blade 106 h, thenotch is created after the operations of shaping the tubular uprights 3,3′, 4 and 4′. Before the use of this blade, the plate 10 presents acontinuity in the zones 8, 8′ and 8″ of join between the uprights 3 and4. In other words, the incorporation of the blade 106 h on the jaw 106enables the notch 19 to be made just before the use of the tool 200,which avoids too great stresses at the level of the line 18 duringproduction of the tubular parts. In practice, the blade 106 h may bewith double cutting edge, with the result that it makes it possible tocut out the plate 10 along two parallel lines 28 and 29 definingtherebetween the notch 19 up to a bore 30 previously made in the plate10. Of course, the jaws 103, 104 and 105 may also be equipped withblades forming shears.

As a function of the geometry of the frame 2, i.e. as a function, inparticular, of the length of the uprights and of their cross-section,different sets of jaws 103 to 106 and of cradles 113 to 116 may be used,the jaws and the cradles being placed in position jointly in theinstallation 100, which makes it possible to effect a rapid standardexchange of the assembly of the bending members of this installation.

The use of the jaws 103 to 106 that may slide on cradles 113 to 116 ofdifferent geometrical axes, such as axes a and c, makes it possible toproduce non-aligned edges, such as the edges 13 and 15, which would notbe possible with conformation jaws articulated on common hinges.

In the Figures, the jaws 103 and 105 and the cradles 113 and 115 havebeen shown as two distinct units. However, it might be question of thesame unit forming bending member at the same time for uprights 3 and 4,as the axis of bend a is the same for the whole edge 12 over the lengthof the plate 10. This is why the plates 103 a and 105 a, on the onehand, and the cradles 103 d and 105 d, might be constituted in onepiece.

When the plate to be bent is thin and in order to avoid a phenomenon ofbuckling of its flanges under the effect of the clamping efforts, it maybe provided to apply the metal sheet against the plates 103 a to 106 aof the jaws 103 to 106 by suction or by magnetic attraction. To thatend, the jaws 103 to 106 may be provided with channels opening out onthe faces 103 a 1, 103 a 2, 105 a 1 and 106 a 1 and connected to asource of vacuum. The jaws may also be equipped with permanent magnetsor with electro-magnets allowing an immobilization of the flanges 23 to26.

As shown in FIG. 17 for an installation according to a second form ofembodiment of the invention, the jaws, such as jaws 104 may be guided bycradles, such as cradle 114 thanks to telescopic circular segments 1041and 1042 which make it possible, for a given geometry of a cradle 114,to obtain an efficient guiding for a movement F₂₀ of pivoting of the jaw104 about axis c of high amplitude. The segment 1041 is fast with thejaw 104 while it is fitted inside the segment 1042, this segment 1042itself being mobile in abutment against the inner cylindrical surface114 c of the cradle 114.

The segment 1042 is hollow while segment 1041 is solid and of width lessthan the width of the inner volume of the segment 1042.

The segment 1042 is provided with rollers 1042 g disposed along itsinner surface 1042 h and adapted to cooperate with the edge 1041 c ofthe segment 1041. These rollers facilitate the relative slide betweenthe segments 1041 and 1042. Moreover, the edge 1042 c of the segment1042 is also provided with rollers 1042 i adapted to cooperate with theinner surface 114 c of the cradle 114. This facilitates pivoting of thesegment 1042.

According to variants of the invention (not shown), the segment 1041and/or the cradle 114 may be equipped with balls or rollers for slide.

The invention has been described, with reference to the first form ofembodiment, with jaws equipped with ribs of which the end surfaces 103c, 104 c, 105 c and 106 c are provided to slide against the cylindricalsurfaces 113 c to 116 c of the cradles 113 to 116. However, the ribs 103b to 106 b may be equipped with runners incorporating balls or rollers,such balls or rollers being provided to roll on the surfaces 113 c to116 c.

Other modes of guiding the jaws by the cradles may be envisaged.

As is more particularly visible in FIGS. 18 and 19, the angles 201 d to204 e and the jaws 201 f to 204 g of the first form of embodiment may bereplaced by profiles of cross-section substantially in U-form, of whichtwo are shown in FIG. 18, with references 202 m and 203 m. Theseprofiles have a cross-section suitable for receiving the uprights 3 and4′respectively, being fixed on flaps similar to flaps 202 and 203 of thefirst embodiment.

Shims 202 p and 203 p are respectively associated with the sections 202m and 203 m for wedging the uprights 3 and 4′ in position. These shimshave a substantially triangular section with a truncated angle, i.e. infact a trapezoidal section. When the uprights 3 and 4′ have been placedin position in the profiles 202 m and 203 m, the shims are introduced inthe profiles as represented by arrows F₂₀, the geometry of the shims 202p and 203 p being such that they are each provided with a surface 202 g,203 g adapted to rest against certain flanges 24 or 26 of the profiles 3and 4′.

Each shim 202 p or 203 p is provided with a blind hole 202 r, 203 r forreceiving a pin 202 s, 203 s provided to pass through an orifice 202 t,203 t of the profiles 202 m and 203 m. The introduction of the pin 202 sin the orifice 202 t and the blind hole 202 r is represented by arrowsF₂₁. Once the pins 202 s and 203 s are in place in the holes andorifices 202 r, 202 t, 203 r and 203 t, the shims are immobilized on theprofiles 202 m and 203 m and maintain the uprights 3 and 4′ in place.

The invention has been shown with jaws whose inner surfaces 103 a 1 to106 a 1 are planar while the flanges 23 to 26 are also planar. Ofcourse, these surfaces and these flanges may be skew as a function ofthe geometry desired for the uprights 3, 3′, 4 and 4′.

The invention has been shown when employed for manufacturing shadow masksupport frames. Such frames may be made in one, two or four parts, eachpart forming one, two or the four uprights of the frame.

1. Method for making a workpiece (2) comprising at least one tubularsection (3, 3′, 4, 4′) obtained by bending a metal plate (10) along atleast one longitudinal line of bending (12–17) intermediate oppositeedge portions of the metal plate, the method including the steps of:initially pre-bending (F) said plate (10) along at least two lines ofbending (12–17) to thereby define at least two parts (23–26) that arebendable relatively toward one another about the at least twolongitudinal lines of bending; and thereafter applying a bending force(F₁, F₃, F₁₁, F₁₃) to at least one of the at least two parts to urge theat least one part toward another part of the at least two parts suchthat the at least two parts at least begin to define a tubular sectionhaving inner and outer surfaces and wherein the bending force is appliedonly along the outer surface and without any forming member within thetubular section being formed while simultaneously applying a clampingforce (F₇, F₈, F₁₂, F₁₄) with respect to the at least one part to urgethe at least one part toward one of the at least two lines of bendingbuy a bending member (103–106) that engages the at least one part alongan outer surface thereof and such that the clamping force preventsslipping of the at least one part relative to the bending member as thebending member bends the at least one part.
 2. Method according to claim1, wherein the pre-bending is effected by immobilizing the plate on anapron (101) by means of a retractable holding clamp (107).
 3. Methodaccording to claim 1 including an additional step of preparing the sheetfor pre-bending by weakening the metal sheet along the at least twolines of bending (12–17).
 4. Method according to claim 1 wherein theplate has at least two longitudinally disposed sections each havingdifferently positioned lines of bending, and bending the plate (10)along different lines of bending (12–17) along a length thereof, withthe result that tubular sections (3, 3′, 4, 4′) of different crosssectional profiles are formed.
 5. Method according to claim 1 includingbending the plate (10) about at least one virtual geometrical axis (a,b, c) defined by a cooperation of the bending member (103–106) and ofguide means (113–116) associated therewith.
 6. Method according to claim1 wherein the clamping force (F₇, F₈, F₁₂, F₁₄) is directedsubstantially towards an adjacent line of bending (12, 13, 15). 7.Method according to claim 6 clamping force (F₇, F₈, F₁₂, F₁₄) isdirected substantially perpendicular to the adjacent line of bending(12–17).
 8. Method according to claim 1 wherein the clamping force (F₇,F₈, F₁₂, F₁₄) stresses the one of the at least two parts (23, 24, 25 a,26 b) of said plate (10) parallel to itself and perpendicularly to thebending force (F₁, F₃, F₁₁, F₁₃) exerted on the one of the at least twoparts by the bending member (103–106).
 9. Method according to claim 1wherein the clamping force (F₇, F₈, F₁₂, F₁₄) is adapted as a functionof a position of the bending member (103–106).
 10. Method according toclaim 1 including bending a first part (26) of the plate (10) to overlapa second part (25) of the plate and reducing the clamping force (F₁₂)exerted on the second part so that the first and second parts are inelastic abutment against each other, and thereafter welding the firstand second parts together.
 11. Method according to claim 1 wherein theplate (10) is bent about at least one virtual axis (a, b, c) defined asa geometrical axis of a cylindrical cradle (113–116) that is used forguiding said bending member (103–106).
 12. Method according to claim 1wherein the plate (10) is bent about different bending axes (a, b, c)along a length (X–X′) of the workpiece (2) using a plurality of bendingmembers (103–106) guided by cradles (113–116) of different geometricalaxes (a, b, c) corresponding to the bending axes.
 13. Method accordingto claim 4 wherein, after formation of a plurality of tubular sections(3, 3′, 4, 4′), thereafter shaping the plurality of tubular sections asa closed frame (2) by bending joining sections (8, 8′, 8″) between twoadjacent tubular sections, perpendicularly to a length (X–X′) of saidtubular sections.
 14. Method according to claim 1 including a step ofretaining, by suction or magnetic attraction, the at least one of thetwo parts (23–26) of the plate against a part (103 a–106 a) of thebending member (103–106).
 15. Installation for manufacturing a workpiece(2) comprising at least one tubular section (3, 3′, 4, 4′), theinstallation comprising at least one bending member (103–106) forbending a metal plate (10) along a longitudinal line of bending (12–17)of the tubular section without an internal forming structure, saidbending member being equipped with clamping means (103 f–106 f) forexerting on a part (23–26) of said plate a clamping force (F₇, F₈, F₁₂,F₁₄) that prevents sliding of the part (23–26) of plate (10) along saidbending member (103–106), while said bending member is articulated abouta virtual axis (a, b, c) of bend located inside the at least one tubularsection (3, 3′, 4, 4′).
 16. Installation according to claim 15,including a plurality of bending members (103–106) adapted to bend saidplate (10) along distinct lines of bending (12–17), with the result thattubular sections (3, 3′, 4, 4′) of different profiles are formed. 17.Installation according to claim 16, wherein each of said clamping meansis constituted by at least one heel (103 f–106 f) forming a stop forabutment of the plate (10).
 18. Installation according to claim 17,wherein said at least one heel (103 f–106 f) extends over substantiallyan entire length (X–X′) of one of the lines of bending.
 19. Installationaccording to claim 16, characterized in that said clamping meanscomprise a plurality of heels (105 f 106 f) distributed over an entirelength (X–X′) of the lines of bending (12, 15) and separated by spaces(105 g–106 g) for receiving heels used with the bending members bendingthe plate (10) along another line of bending.
 20. Installation accordingto claim 15, including means for varying a position (F₉)of said clampingmeans with respect to the line of bending.
 21. Installation according toclaim 15, including means for adjusting the clamping force (F₇, F₈, F₁₂,F₁₄) exerted by said clamping means (103 f–106 f) on the plate (10). 22.Installation according to claim 15, wherein said bending member(103–106) includes at least one surface (104 a 1–106 a 1) for abutmentagainst said plate (10) and at least one circular-base cylindricalsurface (103 c–106 c) adapted to cooperate with a circular-basecylindrical surface (113 c–116 c) of a cradle (113–116), saidcylindrical surfaces being centered on the virtual axis (a, b, c) ofbend of the plate.
 23. Installation according to claim 22, including aplurality of bending members (103–106) distributed over a length (X–X′)of the plate (10) and each bending member including a cradle (113–116)whose cylindrical surface (113 c–116 c) has virtual axes (a, b, c)parallel and offset with respect to one another.
 24. Installationaccording to claim 22, wherein said cylindrical surfaces aresubstantially of the same radius (R₁₀₃, R₁₁₃, R₁₀₄, R₁₁₄). 25.Installation according to claim 22 wherein each bending member (104) isguided with respect to said surface (114 c) of said cradle (114) bytelescopic segments (1041, 1042).
 26. Installation according to claim25, wherein said cylindrical surfaces (103 c—106 c), said telescopicsegments (1041, 1042) and/or said surface (113 c–116 c) of said cradles(113–116) are equipped with balls or rollers (1042 g–1042 i). 27.Installation according to claim 16 including a tool (200) for formingthe workpiece (2), said tool including elements (201–204) supportingtubular sections (3, 3′, 4, 4′) of said workpiece (2) provided withmeans (201 d–204 d, 201 e–204 e, 201 f–204 f, 201 g–204 g) for receivingand immobilizing the tubular parts, and said means for receiving andimmobilizing being articulated (Y₁, Y₂, Y₃) with respect to one another.28. Installation according to claim 27, wherein said means for receivingand immobilizing (201–204) are adjustable in length (X₁, X₂, X₃, X₄).29. Installation according to claim 27 wherein said means for receivingand immobilizing (201–204) are adapted to form, together, a closedfigure of predetermined geometry corresponding to the geometry of aworkpiece (2) in a configuration of a frame.
 30. Installation accordingto claim 27 wherein at least one of said means for receiving andimmobilizing (203–204) is equipped with a member (202 m or 203 m) withcross-section substantially in a form of a U adapted to receive at leastone shim (202 p, 203 p) for blocking one of said tubular sections (3,3′, 4, 4′) in place therein, and means (202 s–203 s) to immobilize saidat least one shim.
 31. Installation according to claim 15, wherein atleast one (106) of said bending members (105–106) is equipped with ablade (106 h) forming shears for cutting out the plate (10) during itsbending (F₁₃).
 32. Use of a method according to claim 1, formanufacturing a frame (2) for supporting a shadow mask (1) for acathode-ray tube.
 33. Use of an installation according to claim 15, formanufacturing a frame (2) for supporting a shadow mask (1) for acathode-ray tube.